The past few decades has seen an explosion of interest in environmental matters. One consequence of this has been the beginning of a movement away from fossil fuel based energy sources with their attendant effects on pollution. One seemingly viable alternative to such traditional energy sources, especially for automobiles, is the electrochemical fuel cell.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Such cells can operate using various reactants—the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, a solution of methanol, or any other suitable reactant. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
One drawback to current fuel cells is the degradation in a cell's power output over time. Impurities, either from the reactant streams or generated from within the fuel cell as intermediate species during the fuel cell reactions, may be adsorbed or deposited on the surface of the anode and the cathode electrocatalysts. One theory is that these intermediate species block portions of the electrocatalyst and prevents these portions from inducing the desired electrochemical reaction. Such impurities are known as electrocatalyst “poisons” and their effect on electrochemical fuel cells is known as “electrocatalyst poisoning”. Such “poisoning” reduces fuel cell performance by reducing the voltage output from the cell for that cell's current density. The deposit of electrocatalyst poisons may be cumulative—over time, even minute concentrations of poisons in a fuel or oxidant stream may result in a degree of electrocatalyst poisoning.
The sources of such poisons, as mentioned above, are legion. Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel but also typically contain electrocatalyst poisons such as carbon monoxide. Because of such a presence, the fuel stream may be pretreated prior to its direction to the fuel cell. Pre-treatment methods may employ catalytic or other methods to remove carbon monoxide. Unfortunately, pre-treatment methods cannot efficiently remove all of the carbon monoxide. Even trace amounts such as 10 parts per million (ppm) can eventually result in electrocatalyst poisoning.
Fuel cell components and other fluid streams in the fuel cell may also be a source of impurities. As an example, fuel cell separator plates are commonly made from graphite. Organic impurities in graphite may leech out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials. A further possible source of poison is from intermediate products in the oxidation process. For cells that use complex fuels, such as methanol, this is particularly important.
In a similar way, the oxidant stream may also contain or produce impurities that inhibit the electrochemical reaction at the cathode. These impurities may originate within the cathode stream, may be generated in-situ or may originate elsewhere in the fuel cell and be transported to the cathode (e.g. organic impurities from the materials used to construct the fuel cell or unreacted or partially reacted fuel from the fuel stream). When air is used as an oxidant, a wide range of atmospheric impurities that are known to be electrocatalyst poisons, may be present. These may include sulphur containing compounds, nitrogen oxides and so on. Adsorption of impurities or the oxidant with the electrocatalyst can also block the electrocatalyst at the anode. It is known that platinum-containing electrocatalysts can react with oxygen to form hydroxides at high cell potentials that inhibit the fuel cell reaction.
A few methods have been developed which attempt to overcome the electrocatalyst poisoning issue. The anode or cathode may be purged with an inert gas. However, this method involves suspending power generation by the fuel cell. Another approach is that of introducing a “clean” fuel stream containing no carbon monoxide or other poisons to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, such a method is not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is unable to operate at full capacity.
Yet another approach is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed by Gottesfeld in U.S. Pat. No. 4,910,099. Unfortunately, this approach has its own drawbacks, such as parasitic losses from oxygen bleed; undesirable localized exothermic reactions at the anode, and dilution of the fuel stream.
Wilkinson et al in U.S. Pat. No. 6,096,448 discloses periodic fuel starvation of the anode to increase the anode potential. This oxidizes and removes electrocatalyst poisons. Wilkinson describes three methods of accomplishing this fuel starvation: momentary interruption of the fuel supply by closing valves both upstream and downstream of the fuel supply, periodically introducing pulses of fuel free fluid into the fuel supply, and momentarily increasing the electrical load on the cell without increasing the fuel supply.
With each of these methods, the anode potential rises because of fuel depletion at the anode. Unfortunately, none of these methods allow direct control of the anode potential. Furthermore, treatment is applied on a stack basis and hence necessarily causes disruption of stack performance.
The PCT Patent Publication WO 01/01508, by Colbow et al., discloses a method and apparatus for operating an electrochemical fuel cell with periodic reactant starvation. Similar, to the Wilkinson patent, Colbow teaches the oxidant starvation of the cathode portion of the fuel cell while the fuel cell continues to produce power to a particular load.
Both Uribe et al., US Patent Publication US 2001/0044040 A1, and Donohue et al., PCT Patent Application WO 01/99218, have disclosed that a brief, periodic increase the output load to reduce cathode potential below 0.6 V can remove chemisorbed OH at the cathode electrocatalyst surface and increase the output of the fuel cell at high operating voltages. The Patent application of Donahue et al teaches several methods for accomplishing this regeneration at the cathode.
The U.S. Pat. No. 6,339,313, issued to Adams et al., discloses a voltage source coupled across a fuel cell. The current derived from the voltage source increases the anode potential of the fuel cell to remove electrocatalyst poisons. Adams further teaches a controller which is connected to a switch bank. The controller utilizes the switch bank to increase current through at least one fuel cell in a fuel cell stack. The Adams patent discloses that a malfunctioning fuel cell, in the fuel cell stack, may be supplemented or replaced with a voltage source.
Fuel cells have been used as a power source in many applications including in electrical vehicular power plants to replace internal combustion engines and as a residential power source. Proton exchange membrane (PEM) type fuel cells include a “membrane electrode assembly” (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution of the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent damage thereto.
For vehicular and residential applications, it is desirable to use a liquid fuel, preferably a hydrocarbon or alcohol, such as methanol, or gasoline as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide but which also includes small amounts of carbon monoxide which is a catalyst poison.
For PEM fuel cell systems, the reaction within the fuel cell must be carried out under conditions which preserve the integrity of the cell and its valuable polymeric and precious metal catalyst components. Since the anode, cathode and electrolyte layers of the MEA assembly are each formed of polymers, it is evident that the integrity and/or capabilities of such polymers may be adversely affected if exposed to too high a temperature.
Many factors must be controlled within the stack to obtain optimum performance from a PEM fuel cell system. Control of water balance in the membrane and at the electrode surfaces is critical if good performance is to be obtained. If the membrane dries out, the cell resistance increases resulting in a drop in cell voltage and the production of heat which can lead to a negative hydration spiral resulting in cell failure as a result of membrane perforation. On the other hand, if water is not removed properly from the cathode surface as it is produced, the cell can flood resulting in poor system performance. Likewise, poisons can accumulate at the anode and cathode surface resulting in poor performance. There are thus a variety of factors that can result in a drop of cell voltage for a given current. Methods for the control of fuel cells based on the measurement of the voltage of individual fuel cells or groups of fuel cells, as disclosed by Keskula et al, U.S. Pat. No. 6,406,806, relating to fuel cell voltage monitoring and system control, are ineffective since there are a number of factors that can lead to such a voltage decline.
From the above, there is therefore a need for devices and methods which address the issue of electrocatalyst poisoning while avoiding the problems associated with the restorative efforts described above. The present invention seeks to overcome the aforementioned shortcomings by removing the poisoning from fuel cells through connecting a variable resistive means, also termed a variable load, or voltage source in parallel with the cells in a fuel cell stack. Furthermore, the present invention seeks to provide an improved device and diagnostic method for controlling processes within the fuel cell stack to manage the operation of the fuel cells individually or in groups of fuel cells.